1. Field of the Invention
This invention relates to a negative-image signal processing circuit and, more particularly, to a processing circuit for a white-balance adjustment.
The invention further relates to a variable-gamma correction circuit and an image signal processing circuit which uses the gamma correction circuit, especially an image signal processing circuit for a white-balance adjustment and gamma correction.
Further, the invention concerns a peak detector circuit for detecting the maximum or minimum level (peak level) of an input voltage. The peak detector circuit is well-suited for use in the above-mentioned negative-image signal processing circuit.
Still further, the invention relates to an electronic image pick-up apparatus having an electronic shutter function and, more particularly, to an apparatus well suited for picking up a planar negative image.
The variable-gamma correction circuit according to the present invention is applicable not only to a color image (signal) but also to monochrome image (signal). The image signal processing circuit according to a the invention is applicable not only to a negative image (signal) but also to a positive image (signal). However, since the image processing circuit is particularly effective for processing a negative image (signal), its application in this specification will be described mainly in connection with a negative image (signal).
2. Description of the Related Art
Negative-image pick-up is necessary in a system of the type in which an image that appears on a negative film is sensed and either displayed on a large-size display screen or projected on a screen in the form of a negative image or upon being converted to a positive image. This is a new system which has appeared on the scene as a replacement for the optical overhead projector and finds use in various meetings, lectures, etc. Since a video signal obtained by picking up a negative image has characteristics different from those of a video signal obtained by picking up a positive image, the video signal cannot be handled in the same manner.
FIG. 11 illustrates an example of tone characteristics (alogarithmic representation) of a negative film. Specifically, FIG. 11 shows the relationship between incident light quantity when a negative film is sensitized, and the development density of the negative film after it has been developed. Density is lowest at a portion A where the film has not been sensitized at all (a portion where light has been completely shut out) and highest at a portion B that has been sensitized completely. The luminance range of the sensed image is not the range from portion A to portion B but rather the luminace range of the sensed image is the range from the darkest portion to the brightest portion of the sensed image, as indicated by the usable range C in FIG. 11. Accordingly, the darkest portion of the sensed image must be the black level of the video signal, and the brightest portion of the sensed image must be the white level of the video signal. This means that it is necessary to detect the upper and lower limits of the usable range C.
In a case where the negative image is a color image, the color-tone characteristics of the three primary colors R, G and B which constitute this color differ from one another, as illustrated in FIG. 12. Moreover, the used ranges (indicated by the bold lines) of these color-tone characteristics also differ from one another. This is a significant problem. When color-tone characteristics differ depending upon the color, the half-tones of the reproduced image become colored. When the used ranges differ, color balance cannot be achieved. This problem arises also with regard to the complementary colors of yellow, magenta and cyan.
FIG. 13 illustrates a negative-image signal processing circuit according to the prior art. An image pick-up apparatus 70 such as a camera (a video camera or a still-video camera, etc.) produces color signals G, R and B representing the three primary colors. The color signals R and B are applied to variable-gain amplifier circuits 85, 86, respectively, which perform a white-balance adjustment by a well-known method. In a case where a negative image is sensed, an adjustment is carried out by the white-balance adjustment in such a manner that the peak levels of the negative-image signal (which level is referred to as a "black peak level"), namely the black peak levels obtained when a reversal is made from negative to positive, agree for the three primary-color signals G, R and B.
The color signals G, R and B obtained as a result of the white-balance adjustment are applied to gamma correction circuits 91, 93, 95, respectively, and to inverter circuits 71, 73, 75, respectively, where the signals are inverted from negative-image signals to positive-image signals. The resulting positive-image signals are applied to respective blanking mixer circuits 72, 74, 76 which superimpose a blanking signal BLK on these signals during their blanking intervals. The resulting signals are applied to gamma correction circuits 92, 94, 96, respectively. The same gamma correction curve is set in each of the gamma correction circuits 91, 93, 95 of the positive-image system. Gamma correction curves are set in the gamma correction circuits 92, 94, 96 of the negative-image system in dependence upon the tone characteristics of G, R and B. The arrangement is such that the tone characteristics after the gamma correction will coincide with the three primary colors of G, R and B.
Changeover switches 101, 102, 103 are provided for respective ones of the color signals G, R, B. Each changeover switch is adapted to switch between the gamma-corrected color signal of the positive system and the gamma-corrected color signal of the negative system. The output color signals G, R and B from the respective changeover switches 101, 102 103 are applied to a matrix circuit 83, whereby the signals are converted into a luminance signal Y and color-difference signals R-Y, B-Y. These signals Y, R-Y and B-Y are converted into an NTSC-format video signal by an encoder 84, which delivers the video signal as an output signal.
In this circuit according to the prior art, gamma correction curves conforming to the tone characteristics of the respective colors are set in the gamma correction circuits 92, 94, 96 of the negative system, and the tone characteristics of the respective color signals following gamma correction are in agreement. As a result, the aforementioned problem of half-tone coloring does not arise.
However, in the white-balance adjustment, the gains of the variable-gain amplifier circuits 85 and 86 are merely adjusted in such a manner that only the black peak levels of the color signals G, R and B coincide. Consequently, in the processing for the reversal from negative to positive, the black peak levels in terms of a positive image coincide but the white peak levels for the positive image do not. The problem that results is an inappropriate white balance.
In the conventional circuitry described above, the three-types of gamma correction circuits 92, 94, 96 in which the different gamma correction curves have been set are required in order to bring the tone characteristics of the three primary colors G, R and B into line. The result is a complicated circuit arrangement.
In general, a variable-gamma correction circuit is useful in varying tone characteristics in line with the subject photographed and personal preference. In order to realize variable gamma characteristics, however, a circuit arrangement of considerable complexity is required. Providing a plurality of such variable-gamma correction circuits would result in a much more complicated circuit arrangement,
A peak detector circuit is useful and widely employed in many electrical circuits and electrical devices. For example, in fields wherein video signals are handled, detecting the peak level of a video signal is important for the purpose of white-balance adjustment and monitoring signal levels.
FIG. 14 illustrates an example of a peak detector circuit according to the prior art. As shown in FIG. 14, an input voltage is clamped by a clamping circuit 113 in such a manner that the reference level thereof attains a predetermined level, after which the clamped signal is fed into an operational amplifier 112. The output of the operational amplifier 112 charges a holding capacitor 110 via a protecting resistor 115 and diode 111. The maximum level of the input voltage is thus held by the capacitor 110. A switching circuit 114 for resetting purposes is connected across the holding capacitor 110.
In the conventional peak detector circuit of this kind, a diode 111 for preventing reverse current is connected between the operational amplifier 112 and the holding capacitor 110. As a result, the voltage held by the capacitor 110 attains a value lower than the maximum voltage of the input voltage by an amount equivalent to the forward voltage V.sub.F (ordinarily several hundred millivolts) of diode 111. Thus, an error, which corresponds to the foward voltage V.sub.F is produced. Moreover, since the forward voltage VF varies by several hundred millivolts depending upon temperature, highly accurate peak detection cannot be achieved.